After multiple years removed from posting in Karlie's thread and reviewing everything related to this case, I still can't come up with any other solution than Karlie left on her own and the most likely scenario is given where her stepmother and father lived is she walked off the beaten path not wanting to be seen by her stepmom.
At that point, she likely got lost trying to find her next destination or return home and had no phone or means to communicate with anyone and IMO it's probable she succumbed to the elements and/or starvation. I have never believed she was abducted, nor do I believe the stepmother harmed her.
Sadly and tragically, I think Karlie was scared and still under the influence of whatever drugs she consumed the previous night at the party - which I think included something much more than just marijuana/THC. Maybe she knew that, maybe she didn't but she consumed it and was dealing with the coming down/after effects.
I think in her inebriated state, she thought she'd be in huge trouble and tried to run away. Perhaps she was trying to 'scare' her stepmom and father so they wouldn't discipline her as much. Or perhaps her logical brain just wasn't working properly because of the drugs. In any event, I think she wandered off voluntarily and got lost. Most tragically, I don't think this is one that will ever be solved.